The long-term goal of this work is to understand the dynamics of DNA transposable elements in natural populations. In 1983-1986, Dr. Boussy discovered a dramatic latitudinal cline in Australia in phenotypic traits associated with the P transposable element system in Drosophila melanogaster. The clinal pattern is presumed to be a transient, unstable condition, and is thus an opportunity to study a major evolutionary transition occurring on a short time scale. This project will assess changes in the cline using 205 isofemale lines of D. melanogaster collected in 1991 by Dr. Woodruff, will test ideas about its structure and formation, and will resample the cline in 1996 to further study its dynamics. The 1991 lines will be tested for P element activity and self-regulatory ability (gonadal dysgenesis assays) and for genomic numbers of types of P elements. These data will be compared to those from 1983-1986 to detect changes in individual populations and in the clinal pattern. Population dynamics theory of P elements suggests that, after an invasion phase dominated by transposition, populations should tend to quasi- equilibria of element types and numbers due to primarily to self- regulation by the elements. The Australian populations seem to be in quasi-equilibria; we will evaluate the types of self-regulation among the lines with different phenotypes. Current theory deals only with single population dynamics. We will test whether gonadal dysgenesis could occur upon migration of individuals between populations differing strongly in their phenotypes, a key point if more realistic models are to be developed incorporating multiple populations with migration between them. We hypothesize that the clinal pattern could have been established by two introductions of flies differing strongly in their P element-associated traits, one into the north and one into the south, with subsequent migration and mixing causing the range of intermediate phenotypes found along the eastern coast. We will test whether populations formed by mixing different proportions of extreme flies will evolve to the range of phenotypes seen along the cline. Collections in 1996 will provide a valuable third look at changes in individual populations and the clinal pattern as a whole. Sampling will focus on regions of transition between phenotypes, and gonadal dysgenesis and molecular data will be compared with those from the 1983-1986 lines and 1991 lines. The latitudinal cline in P element-associated traits in eastern Australia represents a unique natural experiment. This research will develop molecular, phenotypic, temporal and geographical data on the P transposable element in natural populations of D. melanogaster.